Skylight systems have previously been provided that are capable of providing the majority of the lighting needs for various flat roof commercial buildings. In such systems, the skylight may convert the excess solar energy that is not needed for illumination into thermal energy that can be used for process hot water, space heating, and solar cooling. Solar cooling apparatus, such as absorption chillers and liquid desiccant dehumidifiers, typically require between 1.3 and 1.7 units of thermal energy to provide one unit of cooling to the building. An economically designed energy managing skylight system may employ skylights that cover only about 5 to 6% of the roof area. Even at relatively high thermal efficiencies, this only provides about one fourth of the thermal energy needed to cool the entire building area below the skylights.
In order to serve the entire building, and to make the best use of the roof as an energy resource, it is preferable to provide supplemental thermal energy in addition to what is generated by such energy managing skylights. It is possible to supplement the thermal energy using conventional solar thermal collectors. However, the cost of installation per unit of thermal energy generated by conventional thermal collectors is much higher than that generated by previously presented energy managing skylights, and the overall project economics can be significantly degraded. Another option is to use a fossil fuel such as natural gas as the supplemental heat source. This has the advantage of providing firm capacity during periods of low sunlight, but the expense of the gas backup degrades the project economics and the additional fossil fuel use works against one of the main product objectives of being a primary renewable energy source. Thus, there is an ongoing need for a lower-cost method of generating thermal energy at a sufficient temperature to drive solar cooling equipment that is optimized for the commercial rooftop and seasonal summer operation.
Moreover, the majority of solar panels on the market today are designed to optimize efficiency for a given amount of solar radiation flux and outside air temperature. The drive for higher efficiency results in the use of relatively expensive materials such as copper, aluminum, and glass, as well as optical treatments such as low emissivity absorber coatings and low reflective coatings for cover glass. The higher the thermal collection efficiency for the panel, the higher will be the stagnation temperature which occurs when the module is in full sunlight but there is no liquid flow to pull the heat away. Stagnation temperatures between 350 and 400° F. are not uncommon for good quality collectors. These high stagnation temperatures then drive the need for even more expensive materials and components to ensure that the panel does not damage itself in full sunlight.
In summary, there are many other design considerations in addition to a high heat collection efficiency per unit area when providing heat to support year-round space conditioning on a flat roof commercial building.
Virtually all existing solutions make use of some form of fin and tube configuration. That is, solar energy is collected on a flat surface normal to the sun's rays, and the heat is conducted along the surface to a tube through which a working fluid flows. As the heat is conducted along the relatively thin absorber surface, there is a significant temperature drop between the absorbing surface and the working fluid. This temperature drop results in thermal losses of between 12 and 18 percent, because the higher temperature of the absorber surface compared to the fluid temperature results in higher losses to the environment. In addition, an efficient fin design requires creating a good thermal bond between the flat sheet and the fluid tube, which is a significant design challenge that drives up costs and creates failure points. The tubes can be clamped, brazed, soldered, or attached with thermal grease, all of which require substantial manufacturing resources. A solar collector configuration which eliminates heat flow transverse to the sun's rays will effectively have a fin efficiency of 1.0, with a significant overall performance improvement, and have none of the assembly issues.
Further, nearly all current designs place the absorber surfaces directly under the glazing. This causes a direct convective and radiative coupling between the two surfaces, accounting for the majority of the heat loss from the collector. The glazing is typically made of low-iron glass, which has a high light transmissivity of about 90% but which is heavy, is a very poor insulator and so does not maintain more than a few degrees temperature difference across it. Since the emissivity of the glass is high (0.9 or so), in order to limit the radiative heat loss, it is necessary for the absorber to have a low emissivity, along with a high absorptivity. This can be achieved using very thin black coatings such as black chrome, but applying such coatings requires specialized techniques such as vacuum deposition. In addition, many such coatings make use of toxic materials that require special handling, all adding considerable expense to the finished product. A collector design in which the absorber surface is somewhat insulated from the glazing, which uses a glazing with more insulating properties, and which can use simpler absorbing materials such as ordinary black paint, would be of lower cost, of higher efficiency, and more environmentally friendly.
Other solutions to reduce collector cost have recently become available that make use of lower cost materials than the classic flat plate or evacuated tube designs. The most common is the unglazed black plastic “pool heater” solar collector. In this collector design, solar radiation is absorbed directly by the black plastic tubes through which the coolant flows. Because there is a low insulation level between the coolant and the environment, this collector design is typically used for low temperature applications in warm climates such as heating water for swimming pools. There are several fundamental limitations which have thus far prevented the deployment of high efficiency, low cost, polymer (i.e., plastic) collectors.
The first problem is the low melting point of plastics that are sufficiently low in cost to be considered for use as collectors. Adding a glazing layer of glass or plastic material over the collector surface is of course the simplest way to increase the efficiency. However, even one glazing layer over a black plastic collector surface can allow the stagnation temperature to quickly exceed the softening point of the plastic.
Secondly, extruded panels with discrete flow channels must be connected to a header or manifold. A waterproof seal must be made between the irregular shape of the cross section of the end of the extrusion and the fluid carrying tube that forms the manifold. The seal is typically made by making the manifold of the same material as the extruded panel and welding the two materials together. The irregular shape of this welded joint makes the joint difficult to fabricate and prone to leakage with thermal cycles.
Further, all plastics have very low strength and stiffness relative to metals. This makes it difficult for plastic solar panels to contain typical aqueous heat transfer fluid that in ordinary solar thermal systems can reach pressures of 150 PSI. The fluid pressure requirements in ordinary solar thermal systems are driven in large part by the use of water-based coolants such as water glycol mixtures which can boil or create vapor bubbles that can rapidly increase the pressure in the fluid passages. A solar collector that makes use of low-cost and lightweight plastic materials, but that overcomes the issues of the low service temperatures and pressure containment, would have the advantages of low cost and weight without the disadvantages of current plastic collector designs.
Further, solar thermal collector systems for use on pitched roof constructions, such as many residential constructions, which may by way of non-limiting example form a part of a solar hot water system for a residential application, can carry their own unique challenges and design requirements. Typical residential solar thermal systems use a temperature sensor, a controller and a 120V mechanical pump, which not only drive material costs, but also constitute a significant portion of the installation costs since more highly skilled installers are required to run wiring and connect sensors. Usually, permits and inspections are required with such electrical installations, which further drives costs. In order to radically lower the cost of residential solar thermal systems, the system must use lighter and lower-cost raw materials to eliminate expensive components such as pumps, sensors, and controls, and be capable of being installed by relatively low-skilled labor.
One solution to simplify residential solar hot water systems is to use thermosiphoning instead of pumped flow. The biggest factor limiting the widespread adoption of thermosiphon residential solar hot water systems is the fact that the addition of glycol to the water increases the viscosity to the point that there is not adequate thermosiphon potential to overcome the additional pressure drop. Therefore, thermosiphon residential solar hot water systems have only been found practical in very warm climates where freezing conditions are never encountered, such as subtropical and tropical regions. In the US, only southern Florida and southern California are candidates for such systems. A thermosiphon-based system which can operate in all climates including very cold climates would have tremendous advantages.
Another factor in the design of a solar collector for both flat roof, generally commercial buildings and for pitched roof, generally residential buildings is the potential use of the solar collector as a cooling device. In climates with high solar radiation during the day and relatively low humidity at night, a significant amount of building cooling can be achieved by using night sky radiant cooling techniques. One problem with night sky radiant cooling is that the typical cooling heat fluxes are only about 1/10 those which can be achieved during solar collection of sunlight. That is, the night sky radiant cooler rejecting heat at 80° F. into a clear night sky can radiate only a maximum of about 80 W per square meter, compared with maximum heat collection rates of 700 or 800 W per square meter in full sunlight. Also, the cooling effect is generally uncorrelated in time with the cooling loads. Therefore, for a night sky radiant cooling method to be cost-effective, it must either be of extremely low cost or must piggyback to provide a cooling function on top of an existing heat collection function, plus it ideally would have low cost thermal storage as well. Several techniques have been described for night sky radiant cooling, such as flushing the roof surface with water at night, and using relatively low efficiency solar collectors as radiators at night, but these techniques are not practical as yet. The fundamental problem with using solar collectors as radiant cooling devices is that the design of the collector is intended to thermally isolate the fluid from the ambient air and the radiant sky environment. A solar collector with two separate fluid paths that could do double duty as an efficient night sky radiant cooling device could make a significant contribution to cooling flat roof commercial buildings in sunny dry climates.
Thus, there remains a need in the art for a cost effective and efficient solar thermal collector system that may also serve as an efficient night sky radiant cooling device that will avoid the foregoing disadvantages of prior art solar thermal collector systems.